1. Field of the Invention
The invention generally relates to improved Mycobacterium tuberculosis (Mtb) vaccines that are successful in preventing the development of symptoms of tuberculosis, both pre- and post-exposure to Mtb. In particular, the invention provides an improved recombinant Bacille Calmette-Guerin (BCG) subunit-based vaccine in which one or more Mtb antigens and one or more Mtb resuscitation or reactivation antigens are overexpressed, and in which at least a portion of the DosR regulon is up-regulated.
2. Background of the Invention
The current prophylactic (pre-exposure) Mycobacterium tuberculosis (Mtb) vaccine Mycobacterium bovis (M. bovis) BCG, introduced over 60 years ago, efficiently protects against severe disease manifestation in children but fails to prevent the establishment of latent TB or disease reactivation of infection in adolescents and adults. Moreover, essentially all novel Mtb vaccines currently in clinical trials are designed as prophylactic rather than both prophylactic and therapeutic (post exposure) vaccines.
It is believed that Mtb progresses through a series of stages during its infectious cycle in man as a reaction to human immune responses and that each stage is orchestrated by a distinct genetic program which directs the expression of stage-specific antigens. If this concept is valid, then a truly comprehensive tuberculosis vaccine should include antigens representing each stage as well as antigens that are stage-independent. Latent tuberculosis (LTBI/latency) appears to be one such stage and current evidence suggests that Mtb adopts a unique physiological phenotype during latency characterized by bacteriostasis (non-replicating persistence), a switch from aerobic to anaerobic respiration, expression of the α-crystalline small chaperone protein (Acr/HspX) and increased resistance to several mycobacterial antibiotics.
Maintenance of the non-replicating persistence state, believed to be typical of Mtb in latent lesions, appears to depend on the continuous production of Th1 cytokines (IFNγ, IL-12 and TNFα) and nitric oxide and the localization of MTB within stable granulomas. However, the reactivation of latent Mtb infection, characterized by resumption of bacterial replication, inflammation and cavitation, can be promptly precipitated by immunosuppressive regimens (e.g., corticosterioids or TNFα-antagonist) and occurs in 5-10% of latently infected individuals, perhaps due to acquired tolerance to environmental mycobacteria, age, and, more significantly, HIV disease. This common clinical scenario and the proven role of the cellular immune system for the maintenance of latency lead to the conclusion that non-replicating persistence is a metastable phenotype determined by three interacting processes: bacterial replication within latent lesions is constrained by effectors of the cellular immune system; bacteria within latent lesions monitor the production of immune effectors; and, decreased production of immune effectors results in resumption of replication.
To date, no successful vaccines have been developed which confer immunity to infection by Mtb and at the same time treat or prevent the development of symptoms of TB after exposure to Mtb, or as a result of reactivation of latent infection. A recombinant BCG vaccine, engineered to elicit an immune response of this kind, might reduce reactivation rates in persons with subtle degrees of immunosuppression produced, for example, by senescence, diabetes, HIV disease, acquired tolerance to environmental mycobacteria or malnutrition. There is thus an ongoing need to develop new TB vaccines, and it would be particularly useful to develop a vaccine that can be used both prophylactically and for post-exposure treatment.